1. Field of the Invention
The present invention related to a crystal oscillator for surface mounting (hereinafter called a “surface-mounting oscillator”) and, in particular, to a voltage-control type of surface-mounting oscillator of a lower profile in which the reliability thereof has been maintained.
2. Description of Related Art
Because surface-mounting oscillators are compact and light, they are often installed as frequency or time reference sources in portable electronic equipment, by way of example. Concomitant with the recent trend towards thinner electronic appliances has arisen a requirement for devices that are of a lower profile, particularly in the heightwise dimension thereof. One such device is disclosed in the patent document described below.
FIG. 2 is illustrative of a prior-art example of a surface-mounting oscillator, where FIG. 2A is a plan view with the cover removed and FIG. 2B is a vertical section therethrough.
This surface-mounting oscillator has a crystal element 2 and an IC chip 3 housed within a main container 1, and is sealed in and enclosed by a cover 4. The main container 1 is formed of a stack of ceramic layers that are rectangular in plan view and have a concave vertical section. Each end thereof in the longitudinal direction has an inner wall inner wall step portions 5, and there are mounting terminals on the outer surface thereof. It should be noted that the inner wall step portion 5 at one end is divided into two portions 5c and 5d, as shown on the left in FIG. 2A.
The crystal element 2 is of a rectangular shape, such as of an AT cut, by way of example. Each main surface thereof has an excitation electrode 6 with an extraction electrode 7 extending out in the lengthwise direction therefrom from one end portion, on opposite sides of the crystal element 2. An end portion extracted out from each extraction electrode 7 is affixed by an electrically-conductive adhesive 8 to one of the inner wall step portions 5 that has a crystal terminal (not shown). The crystal terminal is connected electrically to the IC chip 3 by an electrically conductive path.
The IC chip 3 integrates components such as an amplifier to form an oscillation circuit, and is disposed parallel to an inner base surface 1e of the main container 1, in close proximity thereto. The non-circuitry side of the IC chip 3 could be affixed to the inner base surface 1e of the main container 1 by an adhesive, by way of example. Bonding wires from IC terminals provided on both sides of the circuitry surface of the IC chip 3 connect to the inner wall step portion 5 at one end and a protruding portion 10. These are connected electrically to mounting terminals (not shown) provided on the outer surface. A capacitor 11a in chip form is provided as a circuit component in line with the IC chip 3. Note that this capacitor 11a is a discrete component because it has a large capacitance of at least 1000 picofarads (pF), making it difficult to integrate. In this case, it provides a bias between the power source and ground, by way of example.
With this surface-mounting oscillator, the crystal element 2, the IC chip 3, the other capacitor 11a are disposed in parallel on the inner base surface 1e of the main container 1. The heightwise dimension can therefore be made smaller, in comparison with a configuration in which the IC chip 3 and the capacitor 11a are disposes on the lower surface of the crystal element 2. This makes the device suitable for use as a surface-mounting oscillator to be installed in thinner electronic appliances. Since the IC chip 3 is connected to the inner wall step portion 5 and the protruding portion 10 by the bonding wires 9 (by a process called wire bonding), the reliability is greater than in configurations in which the connections are by means such as ultrasonic pressure welding using bumps. (See Japanese Patent Laid-Open Publication No. 9-83248.)
However, the above-described surface-mounting oscillator of the prior art raises a problem in that a device in which the heightwise dimension has been reduced has large external dimension in plan view, when configured as a voltage control oscillator, by way of example. In other words, a voltage control oscillator requires circuit components such as a voltage-variable capacitive component and inductor as discrete components (in chip form) that are difficult to integrate, in addition to the capacitor 11a. If these circuit components are disposed horizontally together with the crystal element 2 and the IC chip 3 (in the same plane therewith), the external dimensions in plan view will become greater, raising the problem that this configuration cannot be applied to this type of surface-mounting oscillator.
For that reason, the IC chip 3 and the circuit components such as the capacitor 11a, a voltage-variable capacitive component 11b, and an inductor 11c are disposed on an indented base surface of the main container 1, to form a surface-mounting oscillator in which one end portion of the crystal element 2 is held on the inner wall step portion 5. However, since the IC chip 3 (which is the tallest element, together with the bonding wires 9) and the crystal element 2 are disposed one on top of the other, a problem arises in that the heightwise dimension increases, as mentioned previously.
Note that the voltage-variable capacitive component 11b has a capacitance that changes with the control voltage, and varies the oscillation frequency. The inductor 11c causes the series resonance frequency of the crystal vibrator to decrease and broadens the expanse of anti-resonance frequencies with the series resonance frequency, thus broadening the amplitude variation of the oscillation frequency. That is why the inductor 11c is necessary in the voltage control oscillator. To allow for the increase the loop height of the bonding wires 9 (the allowance for deflection thereof), the IC chip 3 is usually the tallest component after packaging.
An objective of the present invention is to provide a surface-mounting oscillator which is highly reliable but with small dimensions in plan view and heightwise, and, in particular, a surface-mounting oscillator of a voltage control type.